Multiple band frequency selective reflectors

ABSTRACT

Polarization independent resonant elements are arrayed in a common plane to form a frequency selective reflective surface for electromagnetic energy. By employing an interspersed array of multiple frequency elements, sufficiently decoupled to permit independent operation, reflections in multiple bands are possible. This makes it possible to operate the reflector at two widely separated frequencies or, by critical separation in terms of frequency, to operate the reflector as a broadband device.

[ MULTIPLE BAND FREQUENCY SELECTIVE REFLECTORS [75] Inventors: James V.Rootsey; Edward S. Jewell,

both of Sunnyvale, Calif.

[73] Assignee: Philco-Ford Corporation, Blue Bell,

22 Filed: Feb. 15,1973

21 Appl. No.: 332,666

[4 1 Oct. 15, 1974 l/l966 Matson et al. 343/909 7/l968 Williams et al.343/909 Primary Examiner-Eli Lieberman Attorney, Agent, or FirmRobert D.Sanbom; Gail W. Woodward [5 7] ABSTRACT 10 Claims, 7 Drawing Figures[52] US. Cl 343/909, 343/779, 343/837 [51] Int. Cl. H0lq. 19/14 [58]Field of Search 343/756, 909, 779, 837

[56] References Cited UNITED STATES PATENTS 2,84l,786 7/1958 Dicke343/909 3,148,370 9/1964 Bowman 343/909 PATENIEDUCT 1 51974 SHEET 10F 2T kQR PMNEQ MULTIPLE BAND FREQUENCY SELECTIVE REFLECTORS BACKGROUND OFTHE INVENTION Frequency selective reflectors have been used to advantage.in the prior art, particularly in the antenna art. In large parabolicreflector type antennas it has been found expedient to operate theantenna at more than one frequency. It is often not practical to locatea plurality of different-frequency feed assemblies at the reflectorfocus. One solution to the problem is to locate a frequency selectiveplane reflector near the antenna focus so that feeds can be mounted onboth sides of the plane reflector, one at the regular focus and one atthe focus image formed by the plane reflector.

In a typical antenna feed a low frequency feed array is located at thereflector focus and aimed at the reflector. A resonant reflector tunedto a substantially higher frequency and hence transparent to the lowerfrequency is located between the feed and the reflector. A second orhigh frequency feed operating at the frequency of the resonant reflectoris located between it and the parabolic reflector and is aimed away fromthe parabolic reflector. In-effect, the focus of the parabolic reflectorat the high frequency is imaged at the location of the high frequencyfeed. Thus both feeds are effectively located at the parabolic reflectorfocus. The plane reflector must be essentially transparent at onefrequency and highly reflective at a second frequency. In the past suchreflectors have been achieved by polarization selection. That is, apolarization selective reflector is used in conjunction with polarizedfeeds. Such a system will not work with circularly polarized signals orunpolarized signals.

It has been found that if conductive resonant elements (typically,cross-shaped conductive elements), having no polarization preference,are arranged on a dielectric surface, the array of crosses will bereflective at the frequency of resonance and transmissive at frequenciessufflciently removed from resonance. Alternatively if a reflectivesurface is provided with an array of apertures having a resonantcharacter independent of polarization it will be transmissive at thefrequency of resonance and reflective at frequencies sufficientlyremoved from resonance. The degree of resonant transmission orreflection will be a function of the density of resonant elementsinvolved and can be made substantial with reasonable structures.

Attempts to broadband such resonant reflectors or to operate them at twofrequencies have been largely unsuccessful. When similar apertures oftwo different resonances are interspersed on a common surface they tendto couple together to result in a single sharp resonance.

SUMMARY OF THE INVENTION It is an object of the invention to provide anunpolarized resonant reflector which is reflective at two differentfrequencies.

It is a further object to provide a broadband unpolarized resonantreflector.

These and other objects are accomplished by interspersing on thereflector different groups of unpolarized resonant elements, each grouphaving a different resonant frequency. These elements of differentresonant frequency must be sufficiently decoupled to permit selfresonance. If the reflective surface is to be composed of cross-shapedelements, this can be achieved by interspersing high and low frequencycrosses oriented at about 45 with respect to each other. Alternativelythe reflective surface can be formed of an array of crosses interspersedwith an array of rings so as to minimize the coupling between arrays. Ina third embodiment, rings and interspersed crosses are combined withsmaller crosses inside the rings with the smaller crosses oriented atabout 45 with respect to the larger crosses. This gives a tripleresonance effect.

FIG. 1 shows an antenna and feed structure in which the presentinvention may be employed;

FIG. 2 is a front view of the structure of FIG. 1;

FIG. 3 is an enlarged section of a plane reflector of a type known inthe prior art for use in the system of FIGS. 1 and 2;

FIG. 4 is a fragmentary view of the improved reflector structure for twofrequency operation;

FIG. 5 is a graph showing the transmission characteristics of a twofrequency device according to the invention;

FIG! 6 is a fragmentary view of the improved reflector structure usinginterspersed rings and crosses; and

FIG. 7 is a fragmentary view of an improved reflector structure designedfor three frequency operation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIGS. 1 and 2 show a known formof antenna system employing a frequency selective reflecting surface.Par abolic reflector l is provided with two radio frequency waveguideand horn feeds. Waveguide 2 is terminated in a horn 3 which is locatedat the reflector focus. Waveguide 4 is terminated by horn 5 and operatesat a substantially higher frequency. Frequency selective reflector plate6 is secured to horn 5 by means of low loss dielectric rods 7. Plate 6is made to be highly reflective to the energy from horn 5 and highlytransmissive to energy from horn 3. Thus the energy from horn 5 isreflected from plate 6 to illuminate reflector 1 while the energy fromhorn 3 illuminates the same reflector directly. The effect'is as if bothfeed horns were located at the reflector focus.

As shown in FIG. 3 plate 6 comprises a series of metal elements 8mounted on a low loss dielectric substrate 9. These elements arecruciform in shape and act like crossed dipole antennas. The elementsactively reflect electromagnetic energy for which they are approximatelyone half wavelength. Such a structure will reflect energy having anypolarization. By employing a relatively large number of such elements,plate 6 will be largely reflective at the frequency of resonance andharmonics thereof. At other frequencies, and particularly frequencieslower than the fundamental resonance, the plate will be highlytransmissive. The elements in plate 6 can have other shapes. For examplethey may have narrower or wider conductors, with the narrow conductorsresulting in sharper resonances thereby producing narrower operatingbandwidth. Some broadbanding of the elements can be achieved by usingdumbbell shapes or a version of the Maltese cross. Also ring shapes willproduce the desired unpolarized resonance where the periphery of thering establishes a fundamental resonance at one wavelength.

The frequency selective plate can be fabricated in several ways. Thesimplest method useful for low power operation is to construct plate 6from metal coated low loss dielectric stock such as is used in printedcircuit fabrication. The desired metal pattern can be produced byconventional photolithographic techniques wherein the unwanted metal ischemically removed. For high power structures the metal elements areconstructed separately and secured by stand-off insulators to adielectric support plate.

While the above description is directed to a plate that is reflective toresonant frequency energy, an alternative arrangement employs a plateexhibiting resonant transmission. For such structures themetal-dielectric patterns are reversed. For example an array ofcruciform holes (the shape of the dipoles in FIG. 3) is cut into adielectric mounted metal plate, using, for example, thephotolithographic process mentioned above. At the frequency for whichthe holes are resonant, such a structure will be highly transmissive.For nonresonant conditions it will be substantially reflective. If sucha plate were to be used in the FIG. 1 showing, the resonant frequency ofplate 6 would be at the frequency of the energy in waveguide 2. Sincethe energy in waveguide 4 would not be resonant, plate 6 in thisalternative arrangement would be reflective.

It has been found that such resonant plates are difficult to operateover a substantial band of frequencies. As mentioned above, if theresonant elements are made rather wide or are suitably shaped, somebroadbanding will occur but this effect is limited. If crosses having atwo-frequency distribution are interspersed they ordinarily tend tocouple together to produce a single response having a resonance that isintermediate between the two frequencies.

If the pattern of FIG. 4 is employed, two frequency operation of theresonant reflector is feasible. A high frequency pattern is arrayedinside the spaces between elements of a low frequency pattern. Thesmaller crosses are rotated about 45 to minimize cross coupling. Such anarray does in fact show two resonances, one each for the two sizes ofcrosses.

FIG. shows the reflection pattern for an array of elements shaped likethose in FIG. 4. The crosses represent conductive material on a low lossdielectric substrate. When the two resonant frequencies are sufficientlyseparated, two reflection peaks are seen as indicated by the solid line0. Such a reflector is operable at two discrete frequencies. If the tworesonant frequencies are closely spaced, the reflection curve of dashedline b in FIG. 5 occurs. The resonance curves complement each other toproduce a broad flat reflection curve. It has been found that for singleresonance peaks such as shown in curve a the 97 percent reflectionbandwidth is ordinarily less than 10 percent. For the broadband versionof curve b, a 97 percent reflection bandwidth of percent is achievable.This broad banding action is greatly desired in modern communicationssystems and is the preferred mode of practicing our invention.

FIG. 6 shows an alternative pattern of two-frequency resonant elementsthat are sufficiently decoupled to permit discrete or broadbandoperation. The rings are fundamentally resonant to the frequency forwhich their periphery is approximately one wavelength (two halfwavelengths back to back).

FIG. 7 shows a three-frequency resonant structure that permits evengreater broadbanding and constitutes a combination of the structures ofFIGS. 4 and 6. The inner crosses represent the highest frequencyelements and the rings the lowest frequency elements.

The foregoing description has shown the fundamental concepts andapplications associated with resonant surface reflection devices andother equivalents and applications will occur to those skilled in theart. Accordingly, it is intended that the scope of the invention belimited only by the following claims:

We claim:

1. In a resonant electromagnetic energy reflector structure having aplurality of polarization insensitive resonant elements, said elementsbeing in a common plane and in sufficient number to render said planeelectrically active at the frequency of resonance of said elements, theimprovement comprising:

interspersing spaced polarization insensitive resonant elements of aplurality of resonant frequencies, said elements being configured andspatially rotated to minimize the electrical coupling between elementsof different resonant frequencies.

2. The improvement of claim 1 wherein said plurality of resonantfrequencies is two, and said resonant ele' ments comprise crosses of onesize interspersed between crosses of a larger size, said crosses of saidone size being oriented at about 45 with respect to said crosses of saidlarger size.

3. The improvement of claim 1 wherein said plurality of resonantfrequencies is two, and said resonant elements comprise crosses resonantat a first frequence interspersed with rings resonant at a secondfrequency.

4. The improvement of claim 1 wherein said plurality of resonantfrequencies is three and said resonant ele ments comprise rings resonantat a first frequency interspersed between crosses resonant to a secondfrequency and an array of crosses resonant to a third frequency locatedso that each ring encloses a cross, said crosses inside said rings beingoriented at about 45 with respect to said crosses resonant to saidsecond frequency.

5. The improvement of claim 1 wherein said resonant elements compriseconductive forms on an insulating substrate and said elements producefrequency selective energy reflection.

6. The improvement of claim 1 wherein said resonant elements compriseapertures in a conductive surface and said elements produce frequencyselective energy transmission.

7. A resonant electromagnetic energy reflector structure comprising:

a first array of polarization insensitive elements dispersedsubstantially uniformly over a common plane, said elements beingresonant at a first frequency, and

a second array of polarization insensitive elements spaced from saidfirst array and also dispersed substantially uniformly over said plane,the elements in said second array being resonant at a second fre quencyand interspersed uniformly among the elements of said first array, saidreflector structure being characterized in that the elements of saidsecond array are spatially rotated to be sufficiently de-coupledelectromagnetically from the elements of said first array to permitoperation of the reflecarrays comprise crosses, said second arraycrosses being smaller and oriented at about 45 with respect to those ofsaid first array.

10. A resonant electromagnetic energy reflector structure as claimed inclaim 7, wherein said first array comprises crosses and said secondarray comprises rings.

1. In a resonant electromagnetic energy reflector structure having aplurality of polarization insensitive resonant elements, said elementsbeing in a common plane and in sufficient number to render said planeelectrically active at the frequency of resonance of said elements, theimprovement comprising: interspersing spaced polarization insensitiveresonAnt elements of a plurality of resonant frequencies, said elementsbeing configured and spatially rotated to minimize the electricalcoupling between elements of different resonant frequencies.
 2. Theimprovement of claim 1 wherein said plurality of resonant frequencies istwo, and said resonant elements comprise crosses of one sizeinterspersed between crosses of a larger size, said crosses of said onesize being oriented at about 45* with respect to said crosses of saidlarger size.
 3. The improvement of claim 1 wherein said plurality ofresonant frequencies is two, and said resonant elements comprise crossesresonant at a first frequence interspersed with rings resonant at asecond frequency.
 4. The improvement of claim 1 wherein said pluralityof resonant frequencies is three and said resonant elements compriserings resonant at a first frequency interspersed between crossesresonant to a second frequency and an array of crosses resonant to athird frequency located so that each ring encloses a cross, said crossesinside said rings being oriented at about 45* with respect to saidcrosses resonant to said second frequency.
 5. The improvement of claim 1wherein said resonant elements comprise conductive forms on aninsulating substrate and said elements produce frequency selectiveenergy reflection.
 6. The improvement of claim 1 wherein said resonantelements comprise apertures in a conductive surface and said elementsproduce frequency selective energy transmission.
 7. A resonantelectromagnetic energy reflector structure comprising: a first array ofpolarization insensitive elements dispersed substantially uniformly overa common plane, said elements being resonant at a first frequency, and asecond array of polarization insensitive elements spaced from said firstarray and also dispersed substantially uniformly over said plane, theelements in said second array being resonant at a second frequency andinterspersed uniformly among the elements of said first array, saidreflector structure being characterized in that the elements of saidsecond array are spatially rotated to be sufficiently de-coupledelectromagnetically from the elements of said first array to permitoperation of the reflector at discrete frequencies represented by theresonant frequencies of the two arrays.
 8. A resonant electromagneticenergy reflector structure as claimed in claim 7, wherein said tworesonant frequencies are sufficiently closely spaced as to produce areflector having a broad frequency response characteristic.
 9. Aresonant electromagnetic energy reflector structure as claimed in claim7, wherein said first and second arrays comprise crosses, said secondarray crosses being smaller and oriented at about 45* with respect tothose of said first array.
 10. A resonant electromagnetic energyreflector structure as claimed in claim 7, wherein said first arraycomprises crosses and said second array comprises rings.